Direct observations of the mechanical behaviors of the cytoskeleton in living fibroblasts

Structural response of microtubule and actin cytoskeletons to direct intracellular load

Microtubule and actin are the two major cytoskeletal polymers that form organized functional structures in the interior of eukaryotic cells. Although the structural mechanics of the cytoskeleton has been extensively studied by direct manipulations in in vitro reconstitution systems, such unambiguous characterizations inside the living cell are sparse. Here, we report a comprehensive analysis of how the microtubule and actin cytoskeletons structurally respond to direct intracellular load. Ferrofluid-based intracellular magnetic tweezers reveal rheological properties of the microtubule complex primarily determined by filamentous actin. The strain fields of the microtubule complex and actin meshwork follow the same scaling, suggesting that the two cytoskeletal systems behave as an integrated elastic body. The structural responses of single microtubules to contact and remote forces further evidence that the individual microtubules are enclosed by the elastic medium of actin. These results, directly characterizing the microtubule and actin cytoskeletons as an interacting continuum throughout the cytoplasm, serve as a cornerstone for the physical understanding of intracellular organization.

In Situ Mechanics of the Cytoskeleton

Not only for man-made architecture but also for living cells, the relationship between force and structure is a fundamental properties that governs their mechanical behaviors. However, our knowledge of the mechanical properties of intracellular structures is very limited because of the lack of direct measurement methods. We established high-force intracellular magnetic tweezers that can generate calibrated forces up to 10 nN, enabling direct force measurements of the cytoskeleton. Using this method, we show that the strain field of the microtubule and actin meshwork follow the same scaling, suggesting that the two cytoskeletal systems behave as an integrated elastic body. Furthermore, quantification of structural response of single microtubules demonstrates that microtubules are enclosed by the elastic medium of filamentous actin. Our results defining the force-structure relationship of the cytoskeleton serve as a framework to understand cellular behaviors by direct intracellular mechanical measurement.

Effects of targeted lung cancer drugs on cardiomyocytes studied by atomic force microscopy

The epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKIs) has become one of the important targeted drugs for the treatment of non-small cell lung cancer (NSCLC). But the cardiac adverse events (AEs) related to the EGFR-TKI treatment occur frequently. And the cases of TKI-associated cardiac AEs remain poorly understood. In order to study the effects of EGFR-TKIs on cardiomyocytes, atomic force microscopy (AFM) was used to measure and analyze the physical properties of cardiomyocytes under the actions of three drugs (gefitinib, afatinib and osimertinib) with different concentrations. By comparing the height, adhesion, Young's modulus, the amplitude and the time of the contraction and relaxation process, it was found that the changes of the mechanical properties of cells were well correlated with the symptoms of AEs, such as cardiomyocyte hypertrophy, QT prolongation, atrial fibrillation, ejection fraction reductions, and cardiac failure. In addition, osimertinib has the most obvious effect on cardiomyocytes at a low concentration, and gefitinib has the greatest effect with the increase of concentration, while afatinib has the least effect on cardiomyocytes. This provides a new method for screening drugs and exploring the principle of action in the process of cancer treatment at the cellular level.

Tuning the Elasticity of Nanogels Improves Their Circulation Time by Evading Immune Cells

Peptide receptor radionuclide therapy is used to treat solid tumors by locally delivering radiation. However, due to nephro‐ and hepato‐toxicity, it is limited by its dosage. To amplify radiation damage to tumor cells, radiolabeled nanogels can be used. We show that by tuning the mechanical properties of nanogels significant enhancement in circulation half‐life of the gel could be achieved. We demonstrate why and how small changes in the mechanical properties of the nanogels influence its cellular fate. Nanogels with a storage modulus of 37 kPa were minimally phagocytosed by monocytes and macrophages compared to nanogels with 93 kPa modulus. Using PET/CT a significant difference in the blood circulation time of the nanogels was shown. Computer simulations affirmed the results and predicted the mechanism of cellular uptake of the nanogels. Altogether, this work emphasizes the important role of elasticity even for particles that are inherently soft such as nano‐ or microgels.

Elastic wrinkling of keratocyte lamellipodia driven by myosin-induced contractile stress

During actin-based cell migration, the actin cytoskeleton in the lamellipodium both generates and responds to force, which has functional consequences for the ability of the cell to extend protrusions. However, the material properties of the lamellipodial actin network and its response to stress on the timescale of motility are incompletely understood. Here, we describe a dynamic wrinkling phenotype in the lamellipodium of fish keratocytes, in which the actin sheet buckles upward away from the ventral membrane of the cell, forming a periodic pattern of wrinkles perpendicular to the cell's leading edge. Cells maintain an approximately constant wrinkle wavelength over time despite new wrinkle formation and the lateral movement of wrinkles in the cell frame of reference, suggesting that cells have a preferred or characteristic wrinkle wavelength. Generation of wrinkles is dependent upon myosin contractility, and their wavelength scales directly with the density of the actin network and inversely with cell adhesion. These results are consistent with a simple physical model for wrinkling in an elastic sheet under compression and suggest that the lamellipodial cytoskeleton behaves as an elastic material on the timescale of cell migration despite rapid actin turnover.

Dynamic information of the time-dependent tobullian biomolecular structure using a high-accuracy size-dependent theory

As the most rigid cytoskeletal filaments, tubulin-labeled microtubules bear compressive forces in living cells, balancing the tensile forces within the cytoskeleton to maintain the cell shape. The current structure is often under several environmental conditions as well as various dynamic or static loads that can decrease the stability of the viscoelastic tubulin-labeled microtubules. For this issue, the dynamic stability analysis of size-dependent viscoelastic tubulin-labeled microtubules using modified strain gradient theory by considering the exact three-length scale parameter. Viscoelastic properties are modeled using Kelvin-Voight model to study the time-dependent tubulin-labeled microtubules structure. By applying energy methods (known as Hamilton's principle), the motion equations of the tubulin-labeled microtubules are developed. The dynamic equations are based on first-order shear deformation theory (FSDT), and generalized differential quadrature and fourth-order Runge-Kutta methods are employed to find the model for the natural frequencies. The novelty of the current study is to consider the effects of viscoelastic properties, and exact values of size-dependent parameters on dynamic behaviors of the tubulin-labeled microtubules. Considering three-length scale parameters (l₀ = h, l₁ = h, l₂ = h) in this size-dependent theory leads to a better agreement with molecular dynamic (MD) simulation in comparison with other theories. The results show that when the rigidity of the edges is improved by changing the simply supported to clamped supported boundary conditions, the maximum deflection and stability of the living part would be damped much more quickly.Communicated by Ramaswamy H. Sarma.

β-tubulin carboxy-terminal tails exhibit isotype-specific effects on microtubule dynamics in human gene-edited cells

This study used human gene-edited cell models and image analysis to reveal that the tubulin C-terminal tails specifically regulate the dynamics of individual microtubules and coordinate microtubule behavior across the cell.

Microtubules are highly dynamic structures that play an integral role in fundamental cellular functions. Different α- and β-tubulin isotypes are thought to confer unique dynamic properties to microtubules. The tubulin isotypes have highly conserved structures, differing mainly in their carboxy-terminal (C-terminal) tail sequences. However, little is known about the importance of the C-terminal tail in regulating and coordinating microtubule dynamics. We developed syngeneic human cell models using gene editing to precisely modify the β-tubulin C-terminal tail region while preserving the endogenous microtubule network. Fluorescent microscopy of live cells, coupled with advanced image analysis, revealed that the β-tubulin C-terminal tails differentially coordinate the collective and individual dynamic behavior of microtubules by affecting microtubule growth rates and explorative microtubule assembly in an isotype-specific manner. Furthermore, βI- and βIII-tubulin C-terminal tails differentially regulate the sensitivity of microtubules to tubulin-binding agents and the microtubule depolymerizing protein mitotic centromere-associated kinesin. The sequence of the β-tubulin tail encodes regulatory information that instructs and coordinates microtubule dynamics, thereby fine-tuning microtubule dynamics to support cellular functions.

Effects of the cross-linkers on the buckling of microtubules in cells

In cells, the protein cross-linkers lead to a distinct buckling behavior of microtubules (MTs) different from the buckling of individual MTs. This paper thus aims to examine this issue via the molecular structural mechanics (MSM) simulations. The transition of buckling responses was captured as the two-dimensional-linkers were replaced by the three-dimensional (3D) ones. Then, the effects of the radial orientation and the axial density of the 3D-linkers were examined, showing that more uniform distribution of the radial orientation leads to the higher critical load with 3D buckling modes, while the inhomogeneity of the axial density results in the localized buckling patterns. The results demonstrated the important role of the cross-linker in regulating MT stiffness, revealed the physics of the experimentally observed localized buckling and these results will pave the way to a new multi-component mechanics model for whole cells.

Experimental and computational assessment of F-actin influence in regulating cellular stiffness and relaxation behaviour of fibroblasts

In biomechanics, a complete understanding of the structures and mechanisms that regulate cellular stiffness at a molecular level remain elusive. In this paper, we have elucidated the role of filamentous actin (F-actin) in regulating elastic and viscous properties of the cytoplasm and the nucleus. Specifically, we performed colloidal-probe atomic force microscopy (AFM) on BjhTERT fibroblast cells incubated with Latrunculin B (LatB), which results in depolymerisation of F-actin, or DMSO control. We found that the treatment with LatB not only reduced cellular stiffness, but also greatly increased the relaxation rate for the cytoplasm in the peripheral region and in the vicinity of the nucleus. We thus conclude that F-actin is a major determinant in not only providing elastic stiffness to the cell, but also in regulating its viscous behaviour. To further investigate the interdependence of different cytoskeletal networks and cell shape, we provided a computational model in a finite element framework. The computational model is based on a split strain energy function of separate cellular constituents, here assumed to be cytoskeletal components, for which a composite strain energy function was defined. We found a significant influence of cell geometry on the predicted mechanical response. Importantly, the relaxation behaviour of the cell can be characterised by a material model with two time constants that have previously been found to predict mechanical behaviour of actin and intermediate filament networks. By merely tuning two effective stiffness parameters, the model predicts experimental results in cells with a partly depolymerised actin cytoskeleton as well as in untreated control. This indicates that actin and intermediate filament networks are instrumental in providing elastic stiffness in response to applied forces, as well as governing the relaxation behaviour over shorter and longer time-scales, respectively.

Characterization of Dynamic Behaviour of MCF7 and MCF10A Cells in Ultrasonic Field Using Modal and Harmonic Analyses

Treatment options specifically targeting tumour cells are urgently needed in order to reduce the side effects accompanied by chemo- or radiotherapy. Differences in subcellular structure between tumour and normal cells determine their specific elasticity. These structural differences can be utilised by low-frequency ultrasound in order to specifically induce cytotoxicity of tumour cells. For further evaluation, we combined in silico FEM (finite element method) analyses and in vitro assays to bolster the significance of low-frequency ultrasound for tumour treatment. FEM simulations were able to calculate the first resonance frequency of MCF7 breast tumour cells at 21 kHz in contrast to 34 kHz for the MCF10A normal breast cells, which was due to the higher elasticity and larger size of MCF7 cells. For experimental validation of the in silico-determined resonance frequencies, equipment for ultrasonic irradiation with distinct frequencies was constructed. Differences for both cell lines in their response to low-frequent ultrasonic treatment were corroborated in 2D and in 3D cell culture assays. Treatment with ~ 24.5 kHz induced the death of MCF7 cells and MDA-MB-231 metastases cells possessing a similar elasticity; frequencies of > 29 kHz resulted in cytotoxicity of MCF10A. Fractionated treatments by ultrasonic irradiation of suspension myeloid HL60 cells resulted in a significant decrease of viable cells, mostly significant after threefold irradiation in intervals of 3 h. Most importantly in regard to a clinical application, combined ultrasonic treatment and chemotherapy with paclitaxel showed a significantly increased killing of MCF7 cells compared to both monotherapies. In summary, we were able to determine for the first time for different tumour cell lines a specific frequency of low-intensity ultrasound for induction of cell ablation. The cytotoxic effect of ultrasonic irradiation could be increased by either fractionated treatment or in combination with chemotherapy. Thus, our results will open new perspectives in tumour treatment.

High-Throughput Assessment of Cellular Mechanical Properties

Traditionally, cell analysis has focused on using molecular biomarkers for basic research, cell preparation, and clinical diagnostics; however, new microtechnologies are enabling evaluation of the mechanical properties of cells at throughputs that make them amenable to widespread use. We review the current understanding of how the mechanical characteristics of cells relate to underlying molecular and architectural changes, describe how these changes evolve with cell-state and disease processes, and propose promising biomedical applications that will be facilitated by the increased throughput of mechanical testing: from diagnosing cancer and monitoring immune states to preparing cells for regenerative medicine. We provide background about techniques that laid the groundwork for the quantitative understanding of cell mechanics and discuss current efforts to develop robust techniques for rapid analysis that aim to implement mechanophenotyping as a routine tool in biomedicine. Looking forward, we describe additional milestones that will facilitate broad adoption, as well as new directions not only in mechanically assessing cells but also in perturbing them to passively engineer cell state.

Nanomechanical force transducers for biomolecular and intracellular measurements: is there room to shrink and why do it?

Over the past couple of decades there has been a tremendous amount of progress on the development of ultrasensitive nanomechanical instruments, which has enabled scientists to peer for the first time into the mechanical world of biomolecular systems. Currently, work-horse instruments such as the atomic force microscope and optical/magnetic tweezers have provided the resolution necessary to extract quantitative force data from various molecular systems down to the femtonewton range, but it remains difficult to access the intracellular environment with these analytical tools as they have fairly large sizes and complicated feedback systems. This review is focused on highlighting some of the major milestones and discoveries in the field of biomolecular mechanics that have been made possible by the development of advanced atomic force microscope and tweezer techniques as well as on introducing emerging state-of-the-art nanomechanical force transducers that are addressing the size limitations presented by these standard tools. We will first briefly cover the basic setup and operation of these instruments, and then focus heavily on summarizing advances in in vitro force studies at both the molecular and cellular level. The last part of this review will include strategies for shrinking down the size of force transducers and provide insight into why this may be important for gaining a more complete understanding of cellular activity and function.

A novel assay reveals preferential binding between Rabs, kinesins, and specific endosomal subpopulations

A novel assay based on expressing FRB-tagged candidate vesicle-binding protein reveals that KIF13A and KIF13B bind preferentially to early endosomes, whereas KIF1A and KIF1Bβ bind preferentially to late endosomes and lysosomes.

Identifying the proteins that regulate vesicle trafficking is a fundamental problem in cell biology. In this paper, we introduce a new assay that involves the expression of an FKBP12-rapamycin–binding domain–tagged candidate vesicle-binding protein, which can be inducibly linked to dynein or kinesin. Vesicles can be labeled by any convenient method. If the candidate protein binds the labeled vesicles, addition of the linker drug results in a predictable, highly distinctive change in vesicle localization. This assay generates robust and easily interpretable results that provide direct experimental evidence of binding between a candidate protein and the vesicle population of interest. We used this approach to compare the binding of Kinesin-3 family members with different endosomal populations. We found that KIF13A and KIF13B bind preferentially to early endosomes and that KIF1A and KIF1Bβ bind preferentially to late endosomes and lysosomes. This assay may have broad utility for identifying the trafficking proteins that bind to different vesicle populations.

Astrocyte processes label for filamentous actin and reorient early within the optic nerve head in a rat glaucoma model

PURPOSE

To determine if astrocyte processes label for actin and to quantify the orientation of astrocytic processes within the optic nerve head (ONH) in a rat glaucoma model.

METHODS

Chronic intraocular pressure (IOP) elevation was produced by episcleral hypertonic saline injection and tissues were collected after 5 weeks. For comparison, eyes with optic nerve transection were collected at 2 weeks. Fellow eyes served as controls. Axonal degeneration in retrobulbar optic nerves was graded on a scale of 1 to 5. Optic nerve head sections (n ≥ 4 eyes per group) were colabeled with phalloidin (actin marker) and antibodies to astrocytic glial fibrillary acidic protein and aquaporin 4, or axonal tubulin βIII. Confocal microscopy and FIJI software were used to quantify the orientation of actin bundles.

RESULTS

Control ONHs showed stereotypically arranged actin bundles within astrocyte processes. Optic nerve head actin bundle orientation was nearly perpendicular to axons (82.9° ± 6.3° relative to axonal axis), unlike the retrobulbar optic nerve (45.4° ± 28.7°, P < 0.05). With IOP elevation, ONH actin bundle orientation became less perpendicular to axons, even in eyes with no perceivable axonal injury (i.e., 38.8° ± 15.1° in grade 1, P < 0.05 in comparison to control ONHs). With severe injury, ONH actin bundle orientation became more parallel to the axonal axis (24.1° ± 28.4°, P < 0.05 in comparison to control ONHs). Optic nerve head actin bundle orientation in transected optic nerves was unchanged.

CONCLUSIONS

Actin labeling identifies fine astrocyte processes within the ONH. Optic nerve head astrocyte process reorientation occurs early in response to elevated IOP.

Tensegrity, cellular biophysics, and the mechanics of living systems

The recent convergence between physics and biology has led many physicists to enter the fields of cell and developmental biology. One of the most exciting areas of interest has been the emerging field of mechanobiology that centers on how cells control their mechanical properties, and how physical forces regulate cellular biochemical responses, a process that is known as mechanotransduction. In this article, we review the central role that tensegrity (tensional integrity) architecture, which depends on tensile prestress for its mechanical stability, plays in biology. We describe how tensional prestress is a critical governor of cell mechanics and function, and how use of tensegrity by cells contributes to mechanotransduction. Theoretical tensegrity models are also described that predict both quantitative and qualitative behaviors of living cells, and these theoretical descriptions are placed in context of other physical models of the cell. In addition, we describe how tensegrity is used at multiple size scales in the hierarchy of life—from individual molecules to whole living organisms—to both stabilize three-dimensional form and to channel forces from the macroscale to the nanoscale, thereby facilitating mechanochemical conversion at the molecular level.

Scaffold-free tissue engineering: organization of the tissue cytoskeleton and its effects on tissue shape

Work described herein characterizes tissues formed using scaffold-free, non-adherent systems and investigates their utility in modular approaches to tissue engineering. Immunofluorescence analysis revealed that all tissues formed using scaffold-free, non-adherent systems organize tissue cortical cytoskeletons that appear to be under tension. Tension in these tissues was also evident when modules (spheroids) were used to generate larger tissues. Real-time analysis of spheroid fusion in unconstrained systems illustrated modular motion that is compatible with alterations in tensions, due to the process of disassembly/reassembly of the cortical cytoskeletons required for module fusion. Additionally, tissues generated from modules placed within constrained linear molds, which restrict modular motion, deformed upon release from molds. That tissue deformation is due in full or in part to imbalanced cortical actin cytoskeleton tensions resulting from the constraints imposed by mold systems is suggested from our finding that treatment of forming tissues with Y-27632, a selective inhibitor of ROCK phosphorylation, reduced tissue deformation. Our studies suggest that the deformation of scaffold-free tissues due to tensions mediated via the tissue cortical cytoskeleton represents a major and underappreciated challenge to modular tissue engineering.

Segmentation and morphometric analysis of cells from fluorescence microscopy images of cytoskeletons

We developed a method to reconstruct cell geometry from confocal fluorescence microscopy images of the cytoskeleton. In the method, region growing was implemented twice. First, it was applied to the extracellular regions to differentiate them from intracellular noncytoskeletal regions, which both appear black in fluorescence microscopy imagery, and then to cell regions for cell identification. Analysis of morphological parameters revealed significant changes in cell shape associated with cytoskeleton disruption, which offered insight into the mechanical role of the cytoskeleton in maintaining cell shape. The proposed segmentation method is promising for investigations on cell morphological changes with respect to internal cytoskeletal structures.

Tomography and static-mechanical properties of adherent cells

A tomography approach is used to reconstruct 3D cell shapes and, simultaneously, the shapes/positions of the nuclei within these cells. Subjecting the cells to well-defined microconfinements of various diameters allow for relating the steady-state shapes of cells to their static-mechanical properties. The observed shapes show striking regularities between different cell types and all fit to a model that takes into account the cell membrane, cortical actin, and the nucleus.

LDL decreases the membrane compliance and cell adhesion of endothelial cells under fluid shear stress

Atherosclerosis is an inflammatory disease of large and medium sized arteriole walls that is precipitated by elevated levels of low-density lipoprotein (LDL) cholesterol in the blood. However, the mechanisms that lead to the initiation of atherosclerosis are not fully understood. In this study, endothelial cells (ECs) were incubated with LDL for 24 h, and then the lipid was detected with Oil Red O staining and cholesterol ester was assayed with high-performance liquid chromatography (HPLC). F-actin was examined by fluorescence microscopy and the viscoelasticity of ECs was investigated using the micropipette aspiration technique. Then, a parallel-plate flow chamber device was used to observe the adhesion and retention of ECs under shear stress. The results demonstrated that elevated LDL significantly increased the cellular lipid content and induced the rearrangement of cytoskeletal F-actin. The initial rapid deformability (l/K 1 + l/K 2) was reduced by elevated cellular LDL levels, while membrane viscosity (μ) was increased by LDL accumulation. After treatment with 150 mg L(-1) LDL for 24 h, the adhesion of ECs under fluid shear stress was significantly decreased (p < 0.05). These results suggested that LDL induced cellular lipid accumulation and cytoskeleton reorganization which increased the cellular stiffness and decreased the adhesion of ECs.

Force transduction and strain dynamics in actin stress fibres in response to nanonewton forces

It is becoming clear that mechanical stimuli are crucial factors in regulating the biology of the cell, but the short-term structural response of a cell to mechanical forces remains relatively poorly understood. We mechanically stimulated cells transiently expressing actin-EGFP with controlled forces (0-20 nN) in order to investigate the structural response of the cell. Two clear force-dependent responses were observed: a short-term (seconds) local deformation of actin stress fibres and a long-term (minutes) force-induced remodelling of stress fibres at cell edges, far from the point of contact. By photobleaching markers along stress fibres we were also able to quantify strain dynamics occurring along the fibres throughout the cell. The results reveal that the cell exhibits complex heterogeneous negative and positive strain fluctuations along stress fibres in resting cells that indicate localized contraction and stretch dynamics. The application of mechanical force results in the activation of myosin contractile activity reflected in an ~50% increase in strain fluctuations. This approach has allowed us to directly observe the activation of myosin in response to mechanical force and the effects of cytoskeletal crosslinking on local deformation and strain dynamics. The results demonstrate that force application does not result in simplistic isotropic deformation of the cytoarchitecture, but rather a complex and localized response that is highly dependent on an intact microtubule network. Direct visualization of force-propagation and stress fibre strain dynamics have revealed several crucial phenomena that take place and ultimately govern the downstream response of a cell to a mechanical stimulus.

Investigation of in vivo microtubule and stress fiber mechanics with laser ablation

Laser ablation has emerged as a useful technique to study the mechanical properties of the cytoskeleton in living cells. Laser ablation perturbs the force balance in the cytoskeleton, resulting in a dynamic response which can be imaged. Quantitative measurement of the dynamic response allows the testing of mechanical theories of the cytoskeleton in living cells. This review discusses recent work in applying laser ablation to study stress fiber and microtubule mechanics in living endothelial cells. These studies reveal that molecular motors are major determinants of the mechanical properties of the cytoskeleton in cells.

Mechanotransduction through fibronectin-integrin focal adhesion in microvascular smooth muscle cells: is calcium essential?

It is believed that increased transmural pressure exerts force on vascular smooth muscle cells (VSMCs) and triggers Ca(2+) signaling as an initiating event responsible for the arteriolar myogenic response. However, the mechanisms linking the pressure increase to Ca(2+) signaling are unclear. We have shown previously using atomic force microscopy (AFM) that mechanical force induces a VSMC contractile response when applied to single fibronectin (FN; Sun Z, Martinez-Lemus LA, Hill MA, Meininger GA. Am J Physiol Cell Physiol 295; C268-C278, 2008) focal adhesion sites. This current study seeks to determine whether application of force to single focal adhesions can cause a change in VSMC Ca(2+). Experiments were performed in low passage (p3∼10) as well as in freshly isolated skeletal muscle arteriole VSMCs. AFM-attached microbeads (5 μm) were coated with FN or collagen type I (CN-I) or type IV (CN-IV) and placed on a VSMC for 20 min, resulting in formation of a focal adhesion between the cell and the microbead. In low passage VSMCs, mechanically pulling on the FN-coated beads (800∼3000 pN) did not induce a Ca(2+) increase but did cause a contractile response. In freshly isolated VSMCs, application of an FN or CN-I-coated bead onto the cell surface induced global Ca(2+) increases. However, these Ca(2+) increases were not correlated with the application of AFM pulling force to the bead or with the VSMC contractile responses to FN-coupled pulling. Chelating cytosolic Ca(2+) using BAPTA loading had no negative effect on the focal adhesion-related contractile response in both freshly isolated and low passage VSMCs, while the Rho-kinase inhibitor Y27632 abolished the micromyogenic response in both cases. These observations suggest that, in freshly isolated and cultured VSMCs, application of mechanical force to a focal adhesion does not invoke an acute global Ca(2+) increase. On the other hand, our data support a role for Rho-linked signaling mechanism involved in mechanotransduction leading to focal contraction that is independent of the need for a global increase in VSMC Ca(2+).

Mechanical manipulation of neurons to control axonal development

Cell manipulations and extension of neuronal axons can be accomplished with calibrated glass micro-fibers capable of measuring and applying forces in the 10-1000 μdyne range. Force measurements are obtained through observation of the Hookean bending of the glass needles, which are calibrated by a direct and empirical method. Equipment requirements and procedures for fabricating, calibrating, treating, and using the needles on cells are fully described. The force regimes previously used and different cell types to which these techniques have been applied demonstrate the flexibility of the methodology and are given as examples for future investigation. The technical advantages are the continuous 'visualization' of the forces produced by the manipulations and the ability to directly intervene in a variety of cellular events. These include direct stimulation and regulation of axonal growth and retraction; as well as detachment and mechanical measurements on any type of cultured cell.

Stress transmission within the cell

An outstanding problem in cell biology is how cells sense mechanical forces and how those forces affect cellular functions. During past decades, it has become evident that the deformable cytoskeleton (CSK), an intracellular network of various filamentous biopolymers, provides a physical basis for transducing mechanical signals into biochemical responses. To understand how mechanical forces regulate cellular functions, it is necessary to first understand how the CSK develops mechanical stresses in response to applied forces, and how those stresses are propagated through the CSK where various signaling molecules are immobilized. New experimental techniques have been developed to quantify cytoskeletal mechanics, which together with new computational approaches have given rise to new theories and models for describing mechanics of living cells. In this article, we discuss current understanding of cell biomechanics by focusing on the biophysical mechanisms that are responsible for the development and transmission of mechanical stresses in the cell and their effect on cellular functions. We compare and contrast various theories and models of cytoskeletal mechanics, emphasizing common mechanisms that those theories are built upon, while not ignoring irreconcilable differences. We highlight most recent advances in the understanding of mechanotransduction in the cytoplasm of living cells and the central role of the cytoskeletal prestress in propagating mechanical forces along the cytoskeletal filaments to activate cytoplasmic enzymes. It is anticipated that advances in cell mechanics will help developing novel therapeutics to treat pulmonary diseases like asthma, pulmonary fibrosis, and chronic obstructive pulmonary disease.

Methods for expressing and analyzing GFP-tubulin and GFP-microtubule-associated proteins

Important advances in our understanding of the organization and dynamics of the cytoskeleton have been made by direct observations of fluorescently tagged cytoskeletal proteins in living cells. In early experiments, the cytoskeletal protein of interest was purified, covalently modified with a fluorescent dye, and microinjected into living cells. In the mid-1990s, a powerful new technology arose: Researchers developed methods for expressing chimeric proteins consisting of the gene of interest fused to green fluorescent protein (GFP). This approach has become a standard method for characterizing protein localization and dynamics. More recently, a profusion of "XFP" (spectral variants of GFP) has been developed, allowing researchers straightforwardly to perform experiments ranging from simultaneous co-observation of protein dynamics to fluorescence recovery after photobleaching (FRAP), fluorescence resonance energy transfer (FRET), and subresolution techniques such as stimulated emission-depletion microscopy (STED) and photoactivated localization microscopy (PALM). In this article, the methods used to express and analyze GFP- and/or XFP-tagged tubulin and microtubule-associated proteins (MAPs) are discussed. Although some details may be system-specific, the methods and considerations outlined here can be adapted to a wide variety of proteins and organisms.

A llama-derived gelsolin single-domain antibody blocks gelsolin-G-actin interaction

RNA interference has tremendously advanced our understanding of gene function but recent reports have exposed undesirable side-effects. Recombinant Camelid single-domain antibodies (VHHs) provide an attractive means for studying protein function without affecting gene expression. We raised VHHs against gelsolin (GsnVHHs), a multifunctional actin-binding protein that controls cellular actin organization and migration. GsnVHH-induced delocalization of gelsolin to mitochondria or the nucleus in mammalian cells reveals distinct subpopulations including free gelsolin and actin-bound gelsolin complexes. GsnVHH 13 specifically recognizes Ca(2+)-activated gelsolin (K (d) approximately 10 nM) while GsnVHH 11 binds gelsolin irrespective of Ca(2+) (K (d) approximately 5 nM) but completely blocks its interaction with G-actin. Both GsnVHHs trace gelsolin in membrane ruffles of EGF-stimulated MCF-7 cells and delay cell migration without affecting F-actin severing/capping or actin nucleation activities by gelsolin. We conclude that VHHs represent a potent way of blocking structural proteins and that actin nucleation by gelsolin is more complex than previously anticipated.

Atomic force microscopy probing in the measurement of cell mechanics

Atomic force microscope (AFM) has been used incrementally over the last decade in cell biology. Beyond its usefulness in high resolution imaging, AFM also has unique capabilities for probing the viscoelastic properties of living cells in culture and, even more, mapping the spatial distribution of cell mechanical properties, providing thus an indirect indicator of the structure and function of the underlying cytoskeleton and cell organelles. AFM measurements have boosted our understanding of cell mechanics in normal and diseased states and provide future potential in the study of disease pathophysiology and in the establishment of novel diagnostic and treatment options.

Musculoskeletal mechanobiology: interpretation by external force and engineered substratum

Mechanobiology aims to discover how the mechanical environment affects the biological activity of cells and how cells' ability to sense these mechanical cues is converted into elicited cellular responses. Musculoskeletal mechanobiology is of particular interest given the high mechanical loads that musculoskeletal tissues experience on a daily basis. How do cells within these mechanically active tissues interpret external loads imposed on their extracellular environment, and, how are cell-substrate interactions converted into biochemical signals? This review outlines many of the main mechanotransduction mechanisms known to date, and describes recent literature examining effects of both external forces and cell-substrate interactions on musculoskeletal cells. Whether via application of external forces and/or cell-substrate interactions, our understanding and regulation of musculoskeletal mechanobiology can benefit by expanding upon traditional models, and shedding new light through novel investigative approaches. Current and future work in this field is focused on identifying specific forces, stresses, and strains at the cellular and tissue level through both experimental and computational approaches, and analyzing the role of specific proteins through fluorescence-based investigations and knockdown models.

Controlled intracellular release of peptides from microcapsules enhances antigen presentation on MHC class I molecules

To understand the time course of action of any small molecule inside a single cell, one would deposit a defined amount inside the cell and initiate its activity at a defined moment. An elegant way to achieve this is to encapsulate the molecule in a micrometer-sized reservoir, introduce it into a cell, remotely open its wall by a laser pulse, and then follow the biological response by microscopy. The validity of this approach is validated here using microcapsules with defined walls that are doped with metallic nanoparticles so as to enable them to be opened with an infrared laser. The capsules are loaded with a fluorescent antigenic peptide and introduced into mammalian cultured cells where, upon laser-induced release, the peptide binds to major histocompatibility complex (MHC) class I proteins and elicits their cell surface transport. The concept of releasing a drug inside a cell and following its action is applicable to many problems in cell biology and medicine.

Multi-scale models of cell and tissue dynamics

Cell and tissue movement are essential processes at various stages in the life cycle of most organisms. The early development of multi-cellular organisms involves individual and collective cell movement; leukocytes must migrate towards sites of infection as part of the immune response; and in cancer, directed movement is involved in invasion and metastasis. The forces needed to drive movement arise from actin polymerization, molecular motors and other processes, but understanding the cell- or tissue-level organization of these processes that is needed to produce the forces necessary for directed movement at the appropriate point in the cell or tissue is a major challenge. In this paper, we present three models that deal with the mechanics of cells and tissues: a model of an arbitrarily deformable single cell, a discrete model of the onset of tumour growth in which each cell is treated individually, and a hybrid continuum-discrete model of the later stages of tumour growth. While the models are different in scope, their underlying mechanical and mathematical principles are similar and can be applied to a variety of biological systems.

Mechanical dynamics of single cells during early apoptosis

Dynamic mechanical properties of cells are becoming recognized as indicators and regulators of physiological processes such as differentiation, malignant phenotypes and mitosis. A key process in development and homeostasis is apoptosis and whilst the molecular control over this pathway is well studied, little is known about the mechanical consequences of cell death. Here, we study the caspase-dependent mechanical kinetics of single cells during early apoptosis initiated with the general protein-kinase inhibitor staurosporine. This results in internal remodelling of the cytoskeleton and nucleus which is reflected in dynamic changes in the mechanical properties of the cell. Utilizing simultaneous confocal and atomic force microscopy (AFM), we measured distinct mechanical dynamics in the instantaneous cellular Young's Modulus and longer timescale viscous deformation. This allowed us to visualize time-dependent nuclear and cytoskeletal control of force dissipation with fluorescent fusion proteins throughout the cell. This work reveals that the cell death program not only orchestrates biochemical dynamics but also controls the mechanical breakdown of the cell. Importantly, the consequences of mechanical disregulation during apoptosis may be a contributing factor to several human pathologies through the poorly timed release of dead cells and cell debris.

An atomic force microscope operating at hypergravity for in situ measurement of cellular mechano-response

We present a novel atomic force microscope (AFM) system, operational in liquid at variable gravity, dedicated to image cell shape changes of cells in vitro under hypergravity conditions. The hypergravity AFM is realized by mounting a stand-alone AFM into a large-diameter centrifuge. The balance between mechanical forces, both intra- and extracellular, determines both cell shape and integrity. Gravity seems to be an insignificant force at the level of a single cell, in contrast to the effect of gravity on a complete (multicellular) organism, where for instance bones and muscles are highly unloaded under near weightless (microgravity) conditions. However, past space flights and ground based cell biological studies, under both hypogravity and hypergravity conditions have shown changes in cell behaviour (signal transduction), cell architecture (cytoskeleton) and proliferation. Thus the role of direct or indirect gravity effects at the level of cells has remained unclear. Here we aim to address the role of gravity on cell shape. We concentrate on the validation of the novel AFM for use under hypergravity conditions. We find indications that a single cell exposed to 2 to 3 x g reduces some 30-50% in average height, as monitored with AFM. Indeed, in situ measurements of the effects of changing gravitational load on cell shape are well feasible by means of AFM in liquid. The combination provides a promising technique to measure, online, the temporal characteristics of the cellular mechano-response during exposure to inertial forces.

Understanding sensory nerve mechanotransduction through localized elastomeric matrix control

Background

While neural systems are known to respond to chemical and electrical stimulation, the effect of mechanics on these highly sensitive cells is still not well understood. The ability to examine the effects of mechanics on these cells is limited by existing approaches, although their overall response is intimately tied to cell-matrix interactions. Here, we offer a novel method, which we used to investigate stretch-activated mechanotransduction on nerve terminals of sensory neurons through an elastomeric interface.

Methodology/Principal Findings

To apply mechanical force on neurites, we cultured dorsal root ganglion neurons on an elastic substrate, polydimethylsiloxane (PDMS), coated with extracellular matrices (ECM). We then implemented a controlled indentation scheme using a glass pipette to mechanically stimulate individual neurites that were adjacent to the pipette. We used whole-cell patch clamping to record the stretch-activated action potentials on the soma of the single neurites to determine the mechanotransduction-based response. When we imposed specific mechanical force through the ECM, we noted a significant neuronal action potential response. Furthermore, because the mechanotransduction cascade is known to be directly affected by the cytoskeleton, we investigated the cell structure and its effects. When we disrupted microtubules and actin filaments with nocodozale or cytochalasin-D, respectively, the mechanically induced action potential was abrogated. In contrast, when using blockers of channels such as TRP, ASIC, and stretch-activated channels while mechanically stimulating the cells, we observed almost no change in action potential signalling when compared with mechanical activation of unmodified cells.

Conclusions/Significance

These results suggest that sensory nerve terminals have a specific mechanosensitive response that is related to cell architecture.

Fluorescence proteins, live-cell imaging, and mechanobiology: seeing is believing

Fluorescence proteins (FPs) have been widely used for live-cell imaging in the past decade. This review summarizes the recent advances in FP development and imaging technologies using FPs to monitor molecular localization and activities and gene expressions in live cells. We also discuss the utilization of FPs to develop molecular biosensors and the principles and application of advanced technologies such as fluorescence resonance energy transfer (FRET), fluorescence recovery after photobleaching (FRAP), fluorescence lifetime imaging microscopy (FLIM), and chromophore-assisted light inactivation (CALI). We present examples of the application of FPs and biosensors to visualize mechanotransduction events with high spatiotemporal resolutions in live cells. These live-cell imaging technologies, which represent a frontier area in biomedical engineering, can shed new light on the mechanisms regulating mechanobiology at cellular and molecular levels in normal and pathophysiological conditions.

Vimentin filaments support extension of tubulin-based microtentacles in detached breast tumor cells

Solid tumor metastasis often involves detachment of epithelial carcinoma cells into the vasculature or lymphatics. However, most studies of cytoskeletal rearrangement in solid tumors focus on attached cells. In this study, we report for the first time that human breast tumor cells produce unique tubulin-based protrusions when detached from extracellular matrix. Tumor cell lines of high metastatic potential show significantly increased extension and frequency of microtubule protrusions, which we have termed tubulin microtentacles. Our previous studies in nontumorigenic mammary epithelial cells showed that such detachment-induced microtentacles are enriched in detyrosinated alpha-tubulin. However, amounts of detyrosinated tubulin were similar in breast tumor cell lines despite varying microtentacle levels. Because detyrosinated alpha-tubulin associates strongly with intermediate filament proteins, we examined the contribution of cytokeratin and vimentin filaments to tumor cell microtentacles. Increased microtentacle frequency and extension correlated strongly with loss of cytokeratin expression and up-regulation of vimentin, as is often observed during tumor progression. Moreover, vimentin filaments coaligned with microtentacles, whereas cytokeratin did not. Disruption of vimentin with PP1/PP2A-specific inhibitors significantly reduced microtentacles and inhibited cell reattachment to extracellular matrix. Furthermore, expression of a dominant-negative vimentin mutant disrupted endogenous vimentin filaments and significantly reduced microtentacles, providing specific genetic evidence that vimentin supports microtentacles. Our results define a novel model in which coordination of vimentin and detyrosinated microtubules provides structural support for the extensive microtentacles observed in detached tumor cells and a possible mechanism to promote successful metastatic spread.

Extracellular matrix-specific focal adhesions in vascular smooth muscle produce mechanically active adhesion sites

Integrin-mediated mechanotransduction in vascular smooth muscle cells (VSMCs) plays an important role in the physiological control of tissue blood flow and vascular resistance. To test whether force applied to specific extracellular matrix (ECM)-integrin interactions could induce myogenic-like mechanical activity at focal adhesion sites, we used atomic force microscopy (AFM) to apply controlled forces to specific ECM adhesion sites on arteriolar VSMCs. The tip of AFM probes were fused with a borosilicate bead (2 ~ 5 microm) coated with fibronectin (FN), collagen type I (CNI), laminin (LN), or vitronectin (VN). ECM-coated beads induced clustering of alpha(5)- and beta(3)-integrins and actin filaments at sites of bead-cell contact indicative of focal adhesion formation. Step increases of an upward (z-axis) pulling force (800 ~ 1,600 pN) applied to the bead-cell contact site for FN-specific focal adhesions induced a myogenic-like, force-generating response from the VSMC, resulting in a counteracting downward pull by the cell. This micromechanical event was blocked by cytochalasin D but was enhanced by jasplakinolide. Function-blocking antibodies to alpha(5)beta(1)- and alpha(v)beta(3)-integrins also blocked the micromechanical cell event in a concentration-dependent manner. Similar pulling experiments with CNI, VN, or LN failed to induce myogenic-like micromechanical events. Collectively, these results demonstrate that mechanical force applied to integrin-FN adhesion sites induces an actin-dependent, myogenic-like, micromechanical event. Focal adhesions formed by different ECM proteins exhibit different mechanical characteristics, and FN appears of particular relevance in its ability to strongly attach to VSMCs and to induce myogenic-like, force-generating reactions from sites of focal adhesion in response to externally applied forces.

Rho-ROCK signaling differentially regulates chondrocyte spreading on fibronectin and bone sialoprotein

The mammalian growth plate is a dynamic structure rich in extracellular matrix (ECM). Interactions of growth plate chondrocytes with ECM proteins regulate cell behavior. In this study, we compared chondrocyte adhesion and spreading dynamics on fibronectin (FN) and bone sialoprotein (BSP). Chondrocyte adhesion and spreading were also compared with fibroblasts to analyze potential cell-type-specific effects. Chondrocyte adhesion to BSP is independent of posttranslational modifications but is dependent on the RGD sequence in BSP. Whereas chondrocytes and fibroblasts adhered at similar levels on FN and BSP, cells displayed more actin-dependent spread on FN despite a 16x molar excess of BSP adsorbed to plastic. To identify intracellular mediators responsible for this difference in spreading, we investigated focal adhesion kinase (FAK)-Src and Rho-Rho kinase (ROCK) signaling. Although activated FAK localized to the vertices of adhered chondrocytes, levels of FAK activation did not correlate with the extent of spreading. Furthermore, Src inhibition reduced chondrocyte spreading on both FN and BSP, suggesting that FAK-Src signaling is not responsible for less cell spreading on BSP. In contrast, inhibition of Rho and ROCK in chondrocytes increased cell spreading on BSP and membrane protrusiveness on FN but did not affect cell adhesion. In fibroblasts, Rho inhibition increased fibroblast spreading on BSP while ROCK inhibition changed membrane protrusiveness of FN and BSP. In summary, we identify a novel role for Rho-ROCK signaling in regulating chondrocyte spreading and demonstrate both cell- and matrix molecule-specific mechanisms controlling cell spreading.

Exploring the control circuit of cell migration by mathematical modeling

We have developed a top-down, rule-based mathematical model to explore the basic principles that coordinate mechanochemical events during animal cell migration, particularly the local-stimulation-global-inhibition model suggested originally for chemotaxis. Cells were modeled as a shape machine that protrudes or retracts in response to a combination of local protrusion and global retraction signals. Using an optimization algorithm to identify parameters that generate specific shapes and migration patterns, we show that the mechanism of local stimulation global inhibition can readily account for the behavior of Dictyostelium under a large collection of conditions. Within this collection, some parameters showed strong correlation, indicating that a normal phenotype may be maintained by complementation among functional modules. In addition, comparison of parameters for control and nocodazole-treated Dictyostelium identified the most prominent effect of microtubules as regulating the rates of retraction and protrusion signal decay, and the extent of global inhibition. Other changes in parameters can lead to profound transformations from amoeboid cells into cells mimicking keratocytes, neurons, or fibroblasts. Thus, a simple circuit of local stimulation-global inhibition can account for a wide range of cell behaviors. A similar top-down approach may be applied to other complex problems and combined with molecular manipulations to define specific protein functions.